CHICAGO — An American pediatrician killed when an Afghan security guard opened fire Thursday at a Kabul hospital was dedicated to helping poor children, volunteering in Afghanistan to train young doctors and periodically returning to Chicago to work in a Christian clinic on the city's southwest side.

Dr. Jerry Umanos "was always working to help inner-city kids and trying to help out any needy, poor kids anywhere," said Jeff Schuitema, Umanos' brother-in-law.

The fatal shooting of three Americans on Thursday at Cure International Hospital in western Kabul was the latest in a string of deadly attacks on foreign civilians in the Afghan capital this year. An American nurse also was wounded in the hospital shootings.

"Our families and friends have suffered a great loss, and our hearts are aching," said Jan Schuitema, Umanos' wife, from outside the family home in Chicago on Thursday afternoon. "We don't hold any ill will towards Afghanistan in general or even the gunman who did this. We don't know what his history is."

Angie Schuitema, Umanos' mother-in-law, told The Associated Press on Thursday that she learned of Umanos' death, first from her son and then from her daughter, Umanos' wife, who is a teacher in Chicago. She said her daughter and son-in-law had just taken a two-week vacation in Turkey.

Colleagues in Chicago are heartbroken about the loss of Umanos, 57, who had worked for more than 25 years at Lawndale Christian Health Center in the city, said Dr. Bruce Rowell, medical director of clinical quality at the facility.

"He was ... for many of us on staff, the pediatrician for our very own children," Rowell said at a news conference in Chicago.

"This loss is a great loss for his family, for those of us he worked with as well as for the people of Afghanistan," Rowell said. "He was a loving and caring physician who served all of his patients with the utmost of respect."

Jeff Schuitema said that when Umanos and his wife's children were old enough, the couple quickly moved to Afghanistan.

"Once the kids got to an age where they could leave, they were off," Schuitema said, struggling to compose himself on the phone from his home in suburban Atlanta. "Jerry was trying to get me to come with him for years but I never did."

The Cure International Hospital where the shooting took place is focused on maternity and pediatric care and serves 37,000 patients annually, said Mark Knecht, Cure International's chief financial officer.

Umanos' volunteer work in Kabul involved training residents and medical students interested in international health.

One of those trainees, Johns Hopkins University medical student Evan Russell, met Umanos in Kabul in 2011 while volunteering at the Cure International Hospital and later worked with him on a project called Empowerment Health. Russell said Umanos had been working for years on training programs to teach Afghan women basic health education and skills to provide health services in their communities.

Page 2 of 2 - "Just this morning, he expressed how excited he was that, after years of development with our Afghan partners, we were already on to our second day of training," Russell said in an email to the AP. "Our efforts in the community will continue on, and we remain deeply committed to the mission to which he devoted his life, but Jerry's daily impact on this program, and on so many other people, will be missed forever."

Umanos was a graduate of Wayne State University School of Medicine and completed his residency at the Children's Hospital of Michigan, according to the nonprofit Empowerment Health's website.

Officials at the U.S. health care nonprofit Cure International said they are working closely with authorities investigating the shooting Thursday morning at their Kabul hospital.

Knecht told reporters outside the group's headquarters in Lemoyne, Pa., that it "remains committed to serve the people of Afghanistan."

He also is asking for prayers for "the families of the victims and those affected by the shooting, as well as the peace in Afghanistan."